Syria has again refused to allow aid into the destroyed suburb of Homs amid mounting evidence of human rights abuses, including the torture of victims at a hospital inside the city.

A defiant President Bashar al-Assad said he was determined to go on fighting what he called "foreign-backed terrorism".

"The Syrian people, who have in the past managed to crush foreign plots … have again proven their ability to defend the nation and to build a new Syria through their determination to pursue reforms while confronting foreign-backed terrorism," he said, according to the state news agency Sana.

State television claimed residents were now slowly returning on foot to Baba Amr. It showed men, women and children trudging past ruined and bullet-marked buildings. There was also footage of a cramped tunnel which Damascus says was used to smuggle arms.

But locals said the reports had been fabricated. Speaking via Skype from the Insha'at area, which neighbours Baba Amr, one resident, Sami, said: "No one has tried to go back there." He said the accents of people interviewed on state TV suggested they were from coastal areas and not from Homs. "It makes me laugh when I see state TV," he said. "We know that it is untrue."

A spokesman for the Red Cross said that despite the authorities giving permission for it to deliver aid and medical supplies to Baba Amr last Thursday, they were still being denied access on the grounds of security.

Russia and China have made clear again that they are still standing by the regime in Damascus, while western leaders again ruled out a Libya-style military intervention. The White House said on Tuesday that the president, Barack Obama, was committed to diplomatic efforts to end the violence, saying Washington sought to isolate Assad, cut off his sources of revenue and encourage unity among his opponents. The United States is proposing a new security council resolution demanding an end to the violence in Syria, first by government forces and then by opposition fighters.

Residents who fled Baba Amr spoke of bodies decomposing under rubble, sewage mixing with litter in the streets and a campaign of arrests and executions. "The smell of death was everywhere. We could smell the bodies buried under the rubble all the time," said Ahmad, who escaped to Lebanon, according to agency reports. "We saw so much death that at the end the sight of a dismembered body … stopped moving us."

There was further violence reported across Syria. In Herak, in Deraa province, where the revolt erupted nearly a year ago, residents said armoured vehicles and tanks had massed on the western fringe of the city and in parts of the centre. There were raids reported in the city of Deir al-Zor.

Activists also claimed attacks had continued in Rastan. Video footage emerged apparently showing a staff general who had defected to the Free Syrian Army in protest at the assault on the city. In it, Adnan Qassim Farazat, holding his identity card, said: "I declare my defection from the Syrian army to the Free Syrian Army, because of the artillery bombing against Rastan which is continuing violently." He added: "Houses have been damaged and children and women were killed. This is not the right behaviour of the Syrian army."

Security forces also opened fire on Tuesday on protests in Douma, a suburb north-east of Damascus which was briefly held by the rebels in January, according to reports that could not be verified.

Omer Hamza, an activist in Douma, claimed several tanks and armoured vehicles, and ten busloads of shabiha, or armed thugs, were seen in a village north of Yabrud, between Damascus and Homs. "A few houses were damaged and some people were detained," he said. Four people were killed in Yabrud, he added. Once again three of the bodies were taken by the security forces, he said. A large funeral was held for the fourth victim, Burhan al-Sihli, whose body was recovered.

Secretly shot video footage aired on Monday by Channel 4 showed what it said were Syrian patients tortured by medical staff at a state-run hospital in Homs.

The video, which Channel 4 said it could not independently verify, showed wounded, blindfolded men chained to beds. A rubber whip and electrical cable lay on a table in one ward. Patients showed what looked like signs of severe beatings.

"I have seen detainees being tortured by electrocution, whipping, beating with batons, and by breaking their legs. They twist the feet until the leg breaks," the employee who made the video said.